Participants in the program of medical benefits coverage provided by sections 103.003 to 103.175 who are eligible for Medicare benefits and who are not eligible for the program of medical benefits coverage provided under sections 103.083 to 103.098 to be their primary plan of coverage benefits shall be provided substantially similar benefits provided participants who are not eligible for Medicare benefits. Medical benefits coverage provided under sections 103.003 to 103.175 shall be coordinated with Medicare benefits for participants covered by part A or part B, or both, of Medicare benefits, or for participants eligible for but not covered by part A or part B, or both, of Medicare benefits, reduced by an amount determined by the claims administrator to provide a benefit equivalent to the amount which would be provided on a coordination of benefit basis for such participants if such participants were covered by part A or part B, or both, of Medicare benefits. As used in sections 103.083 to 103.098, the term “Medicare benefits” shall include those medical benefits provided by Title XVIII, A and B, Public Law 89-97, 1965 amendments to the federal Social Security Act (42 U.S.C. § 301, et seq.) and amendments thereto. Any participating member agency having employees or eligible retirees not covered by Medicare shall authorize the plan at its option to enroll those individuals for medical benefits as provided by Title XVIII, A and B, Public Law 89-97, 1965 amendments to the federal Social Security Act whenever they become eligible for such benefits and the plan shall pay the premium for such enrollment on behalf of that person. The Medicare premium amounts shall be included in the rate established by the actuary for providing medical benefits coverage to such a participating member agency. Anyone not authorizing this Medicare enrollment shall be denied coverage.